# Chemical Equilibrium: Law of Equilibrium and Concentration Equilibrium Constant (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Law of Equilibrium and Concentration Equilibrium Constant

For a reversible reaction

At equilibrium, the following ratio of concentration terms always remains constant at a given temperature.

The above relation is known as the law of equilibrium.

For example

At equilibrium, the concentrations of and become constant.

The following ratio of concentration terms always remains constant.

and represent the equilibrium molar concentrations of and respectively and is called the concentration equilibrium constant.

## Pressure Equilibrium Constant

In case of gases their partial pressures can also be used in place of molar concentrations (since the two are directly proportional to each other) in the law of equilibrium.

The new equilibrium constant, , is called the pressure equilibrium constant.

For the reaction between and ,

For the general gas phase reaction

## Relation between and

For a general gas phase reaction at equilibrium

The pressure and concentration equilibrium constants and are

For a gaseous substance i, the ideal gas equation is

and are its partial pressure and amount in a gaseous mixture and V and T are its volume and temperature and R is the gas constant.

It can be written as

is the molar concentration or molarity of ‘i’ expressed in moles per litter.

Using this relation

By using square bracket notation

is the change in the moles of gaseous substances in the reaction and is equal to the difference in the moles of gaseous products and the moles of gaseous reactant.

## Homogeneous Equilibria

### Esterification of Acetic Acid with Ethanol

No gas is involved, therefore expression for is meaningless.

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